1. Field of the Invention
This invention relates to manufacturing control systems for controlling plants manufacturing integrated circuits and more particularly, to systems for improving efficiency of use of tools for manufacturing of integrated circuit parts.
2. Description of Related Art
Workstation Preventative Maintenance (PM) is one of the most frequent activities in the manufacturing control system of an Integrated Circuit (IC) plant. In the past, the time schedules for PM was dominated by technical concerns, rather than by other factors which might be considered. No actions were taken either to evaluate the possible effect of a PM schedule upon the efficiency of operation of the IC plant. Neither were the production issues such as the linearity of the output and waste due to lose of potential manufacturing capacity considered, nor were strategies employed to avoid the impact of the PM schedule.
U.S. Pat. No. 5,446,671 of Weaver et al. for "Look-Ahead Method for Maintaining Optimum Queued Quantities of In-Process Parts at a Manufacturing Bottleneck" monitors potential manufacturing bottlenecks and sets maximum and minimum queue quantities for each product having a bottleneck associated therewith. The processing schedule of a particular lot which in ready to proceed to a bottleneck step is determined based upon an analysis as to whether its maximum and minimum queue limits comply with certain criteria. If not, then the lot has a set flag status associated therewith which delays processing until after all other lots which have a clear flag status have been processed.
U.S. Pat. No. 5,325,304 of Aoki for "Production Instructing Apparatus for Feeding Required Quantities of Materials to Meet an Appointed Delivery Date" shows a system which has a file for storing the state of every product being manufactured and every stock product and a file for storing the production capacity of each producing apparatus for adjusting the feeding of materials to meet delivery dates. Priority lots are given priority in scheduling.